Clint C. Shock, Erik Feibert, and Monty Saunders

Malheur Experiment Station

Oregon State University

Ontario, Oregon, 1995


Nitrogen fertilizer applied through subsurface drip irrigation tape has the potential to be more efficient than nitrogen fertilizer applied broadcast, sidedressed, or water run in a furrow irrigated field. Crop nitrogen applications with drip irrigation could be reduced compared to furrow irrigation as a result of the higher application efficiency. Onion production with subsurface drip irrigation has been tested at the Malheur Experiment Station since 1992. The optimum N fertilization practices for subsurface drip irrigated onions is unknown. The objective of this trial was to determine the optimum N rate for drip irrigated onions to maximize yield and quality. The trial would need to be repeated several years to develop production guidelines.


The trial was conducted on a Owyhee silt loam previously planted to wheat at the Malheur Experiment Station, Ontario, OR. The experimental site had reduced topsoil due to leveling operations several decades ago and had received little chemical fertilizer during the last decade. A soil sample taken from the top foot on March 8, 1995 showed a pH of 7.8, 1.2 percent organic matter, 17 meq per 100 g of soil of cation exchange capacity, 6 ppm nitrate-N, 7 ppm ammonium-N, 20 ppm phosphorus, 429 ppm potassium, 10640 ppm calcium, 469 ppm magnesium, 434 ppm sodium, 1.1 ppm zinc, 8.5 ppm iron, 9 ppm manganese, 1.7 ppm copper, and 0.4 ppm sulfate-S.

On April 5, the field was goundhogged following the broadcast application of 200 lb of phosphate as 0-46-0 and 15 lb of zinc as zinc sulfate. The field was made into 64-inch beds (88-inch centers) and 8 mm black polyethylene drip tapes were laid in a single pass on April 6. In each bed three drip tapes were spaced 24 inches apart and buried 4 inches deep. Each drip tape serviced 3 onion rows. The beds were remade on April 12 using a bed harrow and roller. Onions (cv. Vision) were planted in 9 single rows spaced 8 inches apart in each bed on April 13. Onions were planted with nine Beck Precision Planters (Mel Beck Precision Planters, Nyssa, OR) mounted 8 inches apart on a tool bar. Onions were planted at 200,000 plants/ac (3.21 inches/seed).

The seven N rates ranged from 0 to 300 lb N/ac in 50 lb N/ac increments. The nitrogen for each treatment was split into five equal amounts (Table 1). The N treatments were applied as uran on May 24, June 9, June 16, June 29, and July 7. Treatments were replicated five times and were arranged in a randomized complete block design. Individual plots were 3 beds wide and 40 feet long. Fertilizer solutions were applied through the drip lines with a venturi injector unit (Mazzei injector Model 287) in each plot.

The trial was drip irrigated for 12 hours on April 19, 8 hours on April 22, and 11 hours on April 27 in order to assure uniform emergence. At each pre-emergence irrigation, the wetting front reached just beyond the 2 onion rows that were 8 inches to each side of each drip line. Onions started emerging on May 1.

Soil water potential in each plot was monitored by two granular matrix sensors (Watermark Soil Moisture Sensors Model 200SS, Irrometer Co., Riverside, CA) placed 8 inches below, and one sensor placed 18 inches below one of the onion rows that was 8 inches to the side of a drip tape. The sensors were connected to a datalogger. The datalogger was programmed to read the sensors 8 times per 24 hours and, if necessary, irrigate the trial. The trial was irrigated to maintain a constant average 8-inch soil water potential of -20 kPa by applying 0.06 inch of water up to 8 times in the 24 hour period. The irrigations were controlled by a solenoid valve connected to the datalogger. The pressure in the drip lines was maintained at 10 psi by a pressure regulator in each plot. Drip tape flow rate was 0.3 gal/min/100 ft at 10 psi with emitters spaced 12 inches apart (Turbulent Twin-Wall, Chapin Watermatics, Watertown, NY).

The amount of water applied was measured by a water meter read daily. Irrigations were terminated on September 11. Onion evapotranspiration (Etc) was measured by an AgriMet weather station at the Malheur Experiment Station.

Ten plants from the border rows in each plot were sampled for nutrient analyses on June 28, July 18, August 2, and August 23. The plants were washed, the roots were analyzed for nitrate-N, phosphate-P, K, and sulfate-S, and the leaves were analyzed for micronutrients by Tremblay Consulting of Jerome, Idaho. Ten plants from the border rows in each plot of three replicates were sampled, weighed fresh, dried, weighed and analyzed for nitrogen content on September 2. When the plant tissue indicated P deficiency in onions in some of the plots, phosphoric acid at 3 lb P/ac was applied on August 11, August 16, and August 30 to all of the plots.

The field was sprayed with Roundup at 1.5 pint/ac (10 oz ai/ac) on April 25 to kill emerging weeds. Post-emergence weed control was obtained by the application of Buctril at 10 oz/ac (3.3 oz ai/ac) and Poast at 16 oz/ac (2.9 oz ai/ac) on May 20 and by the application of Goal at 10 oz/ac (1.9 oz ai/ac), Buctril at 12 oz/ac (4 oz ai/ac), and Poast at 16 oz/ac (2.9 oz ai/ac) on May 30, June 10, and July 5.

The onions were lifted on September 25. On September 29 the onions in the central 30 feet of the middle bed in each plot were topped, bagged, and placed into storage. The onions were graded out of storage on December 14. Bulbs were graded according to their diameters: small (<2.25 inches), medium (2.25-3 inches), jumbo (3-4 inches), and colossal (>4 inches). Split bulbs were graded as Number Two's regardless of diameter. Marketable onions were considered perfect bulbs in the medium, jumbo, and colossal size classes. Bulb counts for all replicates of the 100 lb N/ac treatment were made during grading in order to determine the actual plant population.

The soil was sampled in one foot increments down to six feet in each plot before planting and after harvest and analyzed for nitrate and ammonium. The N balances were calculated by subtracting the post harvest accounted nitrogen (crop N uptake plus available soil N after harvest) from the nitrogen supply (available soil N in spring plus fertilizer N plus N from irrigation water plus N from organic matter mineralization). Nitrogen contribution from the irrigation water was estimated to be 2.3 lb N/ac-inch/ac of water infiltration. Nitrogen contribution from organic matter mineralization was estimated by anaerobic incubation at 104 oF for 7 days.

Results and Discussion

Onion emergence was excellent. Onion growth and development continued until irrigation was cut off in early September. The crop suffered hail damage on June 16, June 19, and July 29. Environmental conditions were poor for onion bulb curing; there were several small rainfall events followed with very slow drying. Storage conditions were warmer and more humid than the ideal, especially in November.

The root nitrate concentrations were consistent with the N rates (Figure 1). The estimated critical root nitrate concentrations were very close to the root nitrate profile for the 100 lb N/ac treatment (Figure 1). In spite of low soil nitrate and ammonium levels at planting, and in spite of low nitrate concentrations in the onions receiving no nitrogen fertilizer, onion yields were not very responsive to nitrogen fertilization (Table 1). The limited of response of onion yield to the N rates could have been caused by N supplied from N mineralization.

Water applications over time exceeded crop evapotranspiration (Figure 2) resulting in 37 acre-inches of applied water (irrigation plus precipitation) and 27 acre-inches of Etc. Soil water potential at 8-inch depth remained close to -20 kPa except for a brief period in late August (Figure 3). Datalogger programming problems made it necessary to manually operate the irrigations during parts of the season. The soil water potential oscillations could have been less if the automated irrigation system had run continuously.

By mid December, nitrogen fertilizer was associated with lower total marketable yields out of storage. Increased N fertilizer rates were detrimental in the 1995 drip irrigated onion trial, leading to both increased storage rot and decreased marketable onion yield by mid December (Tables 1 and 2). Trials need to be repeated several years in different fields and different environmental conditions to provide recommendations for growers.

The results of the adjoining trial to determine the optimum plant population for drip irrigated onions indicated that plant populations above 125,000 plants/ac are detrimental to onion yield and quality. The plant population used in this trial (204,930 plants/ac) was probably excessive, contributing to high storage rot at all N rates. Due to the high plant populations used in this trial, the detrimental effects on N fertilizer alone on onion storage quality in 1995 are difficult to determine. Nitrogen fertilizer rates for drip irrigated onions need further examination at plant populations closer to 125,000 plants per acre.

It is difficult to determine the exact causes of high losses in storage in this trial. The 1995 crop was late maturing and had an late irrigation cut off date. The three hail occurrences hurt the crop and may have further delayed maturity. The warmer and more humid storage conditions in November also was a complicating factor in bulb decomposition.

N mineralization supplied substantial amounts of available-N during the season (Tables 3 and 4). N balances were negative for all treatments and increased with increases in N rate (Tables 3 and 4). A negative N balance indicates a probable loss of N through leaching or volatilization. Since water applications plus rainfall were higher than crop water use, the substantial N losses could have been due to leaching. Substantial rainfall events during the 1995 season made it difficult to maintain wet soil for optimum onion production yet avoid soil nitrate leaching. Since water applications were uniform for all plots, the N balances emphasize the losses of N fertilizer and the importance of avoiding over application of N fertilizer.




Table 1. Onion yield response to N rates under drip irrigation. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1995.
N rate N per application  Yield by market grade   Marketable  
#2 Small Medium Jumbo Colossal Total yield medium to colossal Rot
lb N/ac lb N/ac   cwt/ac  cwt/ac  cwt/ac 
0 0 3.8 51.6 201.0 619.4 8.6 884.4 587.0 258.5
50 10 9.9 46.2 190.9 666.0 5.4 918.4 541.3 342.5
100 20 7.3 49.2 187.0 704.8 11.0 959.3 474.9 456.5
150 30 25.5 25.5 169.0 720.9 10.4 951.3 306.6 628.7
200 40 16.3 32.2 165.1 759.5 9.3 982.3 376.7 585.9
250 50 13.9 33.0 164.5 706.1 6.0 923.5 353.3 551.9
300 60 26.1 28.4 154.5 742.9 13.1 965.0 332.3 612.5
LSD (0.05)    ns 16.8 30.5 86.8 ns ns 117.3 139.6

Table 2. Onion market grade distribution response to N rates under drip irrigation. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1995.
N rate Market grade distribution by weight Marketable  
#2 Small Medium Jumbo Colossal medium to colossal Rot
lb N/ac  % 
0 0.4 5.8 22.8 70.0 1.0 66.3 29.3
50 1.1 5.0 20.8 72.5 0.6 59.2 37.0
100 0.8 5.1 19.5 73.5 1.1 50.4 46.6
150 2.7 2.7 17.9 75.5 1.1 31.9 66.4
200 1.6 3.3 16.8 77.4 0.9 38.6 59.4
250 1.5 3.5 17.7 76.6 0.7 38.4 59.7
300 2.7 2.9 16.0 77.0 1.4 34.4 63.5
LSD (0.05)  ns 1.8 3.4 4.9 ns 12.5 13.1

Table 3. Influence of N rate on seasonal available nitrogen accounting in onions and in the soil profile. Malheur Experiment Station, Oregon State University, Ontario, OR, 1995. 
N rate N supply Fall nitrogen accounting Balance*
Pre-plant soil available N (0-6') Fertilizer N N in irrigation water Estimated N mineralization  Fall soil available N (0-6') Plant N recovery Accounted N
lb N/ac ---------- lb/ac ----------
0 276.7 0 78.8 115.5 169.5 160.7 330.2 -140.8
50 276.7 50 78.8 115.5 166.8 149.5 316.2 -204.8
100 276.7 100 78.8 115.5 170.2 163.1 333.2 -237.8
150 276.7 150 78.8 115.5 170.4 166.8 337.3 -283.8
200 276.7 200 78.8 115.5 164.7 181.2 345.9 -325.2
250 276.7 250 78.8 115.5 198.0 172.0 370.0 -351.1
300 276.7 300 78.8 115.5 209.6 169.9 379.5 -391.6
LSD (0.05)          ns ns ns -50.9
* based on the difference between N supplies and fall N accounting.  

Table 4. Influence of N rate on seasonal available nitrogen accounting in onions and in the top two feet of soil. Malheur Experiment Station, Oregon State University, Ontario, OR, 1995.
N rate N supply Fall nitrogen accounting Balance*
Pre-plant soil available N (0-2') Fertilizer N N in irrigation water Estimated N mineralization  Fall soil available N (0-2') Plant N recovery Accounted N
lb N/ac ---------- lb/ac ----------
0 76.2 0 78.8 115.5 45.5 160.7 206.2 -64.3
50 76.2 50 78.8 115.5 43.0 149.5 192.4 -128.1
100 76.2 100 78.8 115.5 57.2 163.1 220.3 -150.2
150 76.2 150 78.8 115.5 66.7 166.8 233.6 -186.9
200 76.2 200 78.8 115.5 51.7 181.2 232.9 -237.6
250 76.2 250 78.8 115.5 61.5 172.0 233.4 -287.1
300 76.2 300 78.8 115.5 66.4 169.9 236.3 -334.3
LSD (0.05)          17.3 ns 28.3 -28.3
* based on the difference between N supplies and fall N accounting.